Entacapone, an inhibitor of catechol-O-methyltransferase (COMT), is a nitro-catechol-structured compound used in the treatment of Parkinson's disease as an adjunct to levodopa/carbidopa therapy. Chemically, entacapone is (E)-2-cyano-3-(3,4-dihydroxy-5-nitrophenyl)-N,N-diethyl-2-propenamide having the following structural formula:

Carbidopa, an inhibitor of aromatic amino acid decarboxylation, is a white, crystalline compound which is slightly soluble in water. Chemically, it is (−)-L-α-hydrazino-(α-methyl-β-(3,4-dihydroxybenzene) propanoic acid monohydrate having structural formula the following structural formula:

Levodopa, an aromatic amino acid, is a white, crystalline compound which is slightly soluble in water. Chemically, it is (−)-L-α-amino-β-(3,4-dihydroxybenzene) propanoic acid having the following structural formula:

Entacapone is a class IV drug under the Biopharmaceutics Classification system and poses problems of low solubility, low dissolution rate and hence low bioavailability.
U.S. Pat. No. 4,963,590 provides a pharmaceutical composition comprising entacapone and pharmaceutically acceptable carrier.
U.S. Pat. Nos. 6,500,867 and 6,797,732 disclose oral solid tablet compositions comprising entacapone, levodopa and carbidopa, or pharmaceutically acceptable salts or hydrates thereof, and a pharmaceutically acceptable excipient. Both these patents disclose that when carbidopa, levodopa and entacapone are mixed together, it results in stability problems and desired therapeutic effect is not achieved. On the other hand, when a substantial portion of carbidopa is separated from levodopa and entacapone, the formulation exhibits better stability and desired therapeutic effect is also achieved.
U.S. Pat. No. 6,599,530 provides an oral compacted composition in the form of a tablet which includes entacapone, nitecapone, or pharmaceutically acceptable salt of entacapone or nitecapone, and croscarmellose sodium in an amount of at least 6% by weight of the composition.
U.S. Application No. 20060222703 describes oral pharmaceutical compositions of entacapone, carbidopa and levodopa with microcrystalline cellulose and starch by simultaneous mixing of all the three actives. The composition is prepared by compaction granulation. The application describes the disadvantages associated with wet granulation technique which includes destabilization of composition and decreased dissolution of levodopa, carbidopa and entacapone due to use of water in the wet granulation method.
Although it is known that micronization or grinding of a substance in the presence of a surfactant or sugar can increase its solubility, these parameters are not always adequate. For example, the bioavailability of micronized progesterone is not adequate and should be improved, for example by dispersion in carnauba wax. Such a technique is described in International Publication No. (PCT) WO 8902742. Thus, it appears that the properties of a substance treated by micronization or grinding, in particular its solubility and its bioavailability, are not predictable and contradictory results may be obtained.
There are numerous prior art references which disclose the use of sugar alcohols like mannitol, sorbitol etc. as fillers in the formulation or as sensory cue agents, i.e. the agents which impart feeling of cooling in mouth in case of orally disintegrating tablets. For example, International Publication Nos. (PCT) WO 2007080601, 2007001086, 2006057912; European Patent Nos, 589981B1, 906089B1, 1109534B1; U.S. Pat. No. 6,328,994, and US Application Nos. 20070196494, 20060240101, and 20060057199. Sugar alcohols like mannitol are employed in the most orally disintegrating formulations and not in the conventional immediate release formulations as sensory cue agents because the orally disintegrating tablets disintegrate in mouth instead of disintegrating in the gastrointestinal tract as in the case of conventional immediate release tablets.